Bank notes and other security documents often incorporate optically variable devices (OVDs) such as diffraction gratings or holographic optical microstructures as security features against copy and counterfeit. The increased use of such devices has been motivated by progress in the fields of computer-based desktop publishing and scanning, which render conventional security print technologies, such as intaglio and offset printing, increasingly susceptible to counterfeit. One way to strengthen security documents against counterfeit is to combine security print with optically variable devices whose structures cannot be replicated by scanners, and which can exhibit optically variable effects such as colour changes by diffraction, movement effects, and distinct switches between images.
For example, holograms are widely used as security features in credit cards as they cannot be reproduced by photographic or photocopying techniques, wherein for example the image beneath the hologram is only visible when the document is tilted. Other OVDs include polymer or laminate microstructures in the form of foils that exhibit colour shifts in transmitted light passing through the foil and/or ambient light reflecting from the foil. Tilting the foil results in a visible colour-shift effect due for example to a laminate microstructure, or Bragg stacking within the foil. Such devices provide particularly useful surface security features in applications where the substrate to which they are applied is flexible or foldable, such as in banknotes.
Security devices, including those comprising OVDs, often take the form or shape of strips or threads. Such threads are traditionally made from a transparent film provided with a continuous reflective metal layer, vacuum deposited aluminium on polyester film being the commonest example. Banknotes incorporating such security devices have been in general circulation in many countries for many years. When such security elements are fully embedded in security paper, and the paper is subsequently printed to provide the security document (e.g. a banknote), the thread cannot be discerned readily in reflected light but is immediately apparent as a dark image when the document is viewed in transmitted light. Such threads are effective against counterfeiting by printing or photocopying, since the optically variable effect cannot be simulated accurately, for example by printing a line on the paper.
Also known in the art is the use of polymer-based films or sheets as an alternative basic substrate for the production of security documents. In some countries such films are used instead of paper-based substrates for bank note production. In comparison to bank notes manufactured using paper substrates, those made from polymer film are highly resilient to tearing, wear and abrasion. As such the bank notes have a longer useable lifespan, and thus may remain in circulation for a significant period of time.
There is a continuing need for improved security devices and features for security documents, as well as improved methods for their manufacture. In particular, the need extends to devices that are difficult to counterfeit, yet relatively inexpensive to manufacture, which are suitable for application to a range of substrate materials including polymer films.